High-frequency ventilation is a respiration therapy which has proved successful especially when respirating neonatal patients. Respiration is carried out in high-frequency ventilation at frequencies of 5-50 Hz or higher, usually as an active support during inspiration and expiration and with a respiration volume that is smaller than, equal to or only slightly larger than the dead space volume. High-frequency ventilation is characterized essentially by the mean pressure (MAP—mean airway pressure), the oscillation frequency and the amplitude (A) of the positive pressure respiration. These characteristics can be found in a pressure (P)-vs.-time (t) diagram.
Among other things, piston oscillators, are known from practice for generating the oscillation frequency necessary for the high-frequency ventilation. They excite the air column in the respiration tubes by means of a rapidly moving cylinder piston. The fresh gas supply is guaranteed by a bias flow system. The oscillations are generated by means of loudspeakers in devices of another design. Fresh gas is added via bias flow in this case as well.
The high-frequency generators known from practice also include so-called flow interrupters. They interrupt the gas flow to the patient at a high frequency and generate an “oscillation” in this manner. The inspiratory gas flow can be interrupted for this by means of a valve or a valve bank, but it is also possible to apply a high constant inspiratory flow and to generate the oscillation by rapidly opening and closing the expiration valve. A jet venturi system (ejector) acting during expiration guarantees active expiration. The necessary “oscillations” can also be brought about by means of a flow cycled according to the inspiration and with an expiration valve operating in antiphase as well as with an ejector, which permits active expiration, and with other methods and devices as well.
A respirator based on high-frequency ventilation is known from U.S. Pat. No. 5,555,880 A. The operator of the device disclosed there or the physician in charge can set the oscillation frequency and the amplitude of the respiration pressure such that the desired tidal volume is approximately applied and a ventilation that is desirable for the patient will thus take place. The set values and the tidal volume that becomes indirectly established herefrom are adapted by the physician in charge manually in case of deviations of the blood gases from target ranges.
The drawback of this prior-art system is that adjustment of the parameters set is necessary even in case of minor changes in the properties of the system, such as changes in the resistance and the compliance of the airways and the lungs of the patient, in case of accumulation of secretion in the airways, in case of changes in the compliance of the tube system or the like. Continuous monitoring of the tidal volume by the physician in charge is therefore absolutely necessary.
The drawback of the device known from U.S. Pat. No. 5,555,880 A is, moreover, that a tidal volume cannot be set directly on the device, but it can be set only indirectly via the combination of other set values such as oscillation frequency and pressure amplitude.